Natural gas produced from subterranean wells has long been an important source of energy and raw material. Natural gas is a clean burning fuel and is particularly applicable for household use and the generation of electricity. It can also be liquefied and used for powering land, seaborne and airborne vehicles. In its role as a raw material, it is commonly converted into fertilizer and a multiplicity of petrochemical products including plastics. At this writing it is abundant in the United States and figures prominently in the mix of energy resources to replace oil which is increasingly expensive and scarce.
Natural gas is usually produced in association with petroleum in so-called ‘oil and gas’ wells. Hydrocarbon wells usually produce water in addition to the oil and gas components. Without lifting liquid to the surface by external means, back pressure is exerted on the reservoir which impedes (or even stops) production into the well. Rod pumping is a frequently used method for lifting liquid to the surface. This system of equipment involves a surface reciprocating machine connected to a positive displacement subsurface pump with a string of sucker rods. Rod pumping has the ability to produce a low back pressure on the reservoir, which allows oil and gas to be produced to the surface at greater rates. While rod pumping is most commonly used, any artificial lift method that is vented is a candidate application for this invention.
FIG. 1A shows a typical oil and gas well being artificially lifted with rod pumping equipment 100. Generally, equipment 100 includes a pump 101 and rods 102, which are reciprocated with a surface pumping unit (not shown). Oil, gas and water comes into the wellbore and the liquids (oil and water and a small amount of gas) are pumped to the surface through tubing 103 and free gas travels to the surface through the annulus between tubing 103 and casing 104. Good production practice strives to vent as much as possible of the free gas upward through the casing-tubing annulus and check valve 105. A gas separator 106 discourages free gas from passing through pump 101 where it would otherwise severely diminish volumetric efficiency of the lift system.
FIG. 1B is a more detailed diagram of gas separator 106. Oil, water and gas enter the wellbore from the reservoir through casing perforations/open hole 107. In general, some of the gas is dissolved in the oil and some is free, i.e. in the gaseous state. Gas separator 106 is designed to separate and vent most of the free gas up the casing-tubing annulus. The free gas moving upward from the casing perforations generally will continue upward in the annulus past the tubing perforations 108.
The liquid (water and oil containing dissolved gas) is forced to move through tubing perforations 108 where it is sucked into pump 101 through suction tube 109. Hopefully, most or all of the free gas is removed by gas separator 106. Several different types of gas separators are available. All have the same purpose, i.e. to vent as much free gas as possible up the casing-tubing annulus. When it reaches the surface, the free gas is mixed with the oil, water and gas that passed through the pump and up the tubing 103 (FIG. 1A). Check valve 105 prevents produced fluids from falling back down the casing-tubing annulus.
Natural gas can also occur in wells in which little associated petroleum and water is produced. These are often termed simply as ‘gas’ wells. FIG. 1C shows a typical gas well. A packer 110 is typically used to seal-off the annulus and produce a better flow regime for gas and liquids to reach the surface.
When multiple gas-producing wells are producing into a single facility, for example a tank battery, allocating the amount of gas produced by the individual wells is important for calculating royalties, and so on. However, as discussed further below, allocating production between wells is a non-trivial process and is subject to a number of sources of errors.